Dual Frequency Radar
One such method is that of dual frequency radars. This requires the use of two wavelengths, one long enough for Rayleigh scattering, the other short enough for non-Rayleigh, or Mie, scattering to occur in the presence of large hydrometeors such as hailstones. Typical wavelengths are 10 cm and 3 cm (S-Band and X-Band). If the radar reflection in dBZ is similar for both wavelengths then the echoes are from small hydro-meteors but if the 3 cm reflection is less than the 10 cm large particles are present.
This is indicative of hail because there are rarely large rain drops; they are unstable and break-up as they fall.
However errors can occur with this method due to the difference in attenuation at the two frequencies, the shorter wavelength will suffer more attenuation. Correction algorithms may be used but hail, rain, dry and wet snow all have different attenuation rates and this can introduce a distortion of the results leading to false hail declaration.